Photosensitive elements (for converting incident radiant energy into an electrical signal) are commonly formed into arrays for use in applications such as x-ray imagers and facsimile devices. Photosensitive elements, such as photodiodes, are formed in conjunction with necessary control or switching elements, such as thin film transistors (TFTs), in relatively large area arrays of such elements.
Photosensitive elements of the general type to which the present invention is applicable are described in the U.S. Pat. No. 5,233,181 of Robert F. Kwasnick and Jack D. Kingsley entitled "Photosensitive Element With Two Layer Passivation Coating", issued Aug. 3, 1993, and assigned to the same assignee as the present invention.
The photodiodes in the photodiode array are addressed by scan lines (rows) which switch the gate voltage of the TFTs associated with each photodiode allowing the charge collected on the photodiode to be transferred to columns of data lines which are connected by external connectors to read out circuitry. A common electrode layer at predetermined potential biases all of the photodiodes in parallel. The photodiode array is formed on a substrate and the scan lines, data lines and common electrode are electrically connected by contact fingers from the outside edge of the array to contact pads at the ends of the contact fingers. External connectors, typically flexible connectors with conducting lines on them, are pressed into contact with the contact pad surfaces to connect to and address the photodiode array.
The imager can be made sensitive to x-ray radiation by formation on the surface of, or coupling to, an X-ray sensitive scintillator. For environmental protection, the scintillator is sandwiched between a cover plate and the imager with the cover plate attached to the imager by a gasket adhering to both the cover plate and imager surfaces.
The contact pads differ from the contact fingers by necessarily having a conducting material on the exposed pad surface to which electrical contact is made. The conducting material is usually the common electrode material such as indium tin oxide (ITO) or a related transparent conducting material. The ITO connects to the underlying conducting materials which in turn connect to the array through vias formed in the dielectric layers. The vias are formed in the TFT passivation dielectric which typically is plasma enhanced chemical vapor deposition (PECVD) deposited silicon oxide (SiOx). It is desirable to avoid exposed conducting material in the contact finger region (the region between the contact pads and the array) of the imager because it can lead to excessive surface leakage between the contact fingers, corrosion, and degradation of imager performance.
In devices such as that described in the aforementioned U.S. Pat. No. 5,233,181, a two layer diode passivation dielectric is included in which the bottom layer is commonly PECVD silicon nitride (SiNx). Because adhesion of gasket material is better to SiOx than to SiNx, in such devices the SiNx is removed from the contact finger and contact pad region. It has been found that defects may occur if the SiNx is removed over the contact finger region by wet etching due to attack of the underlying SiOx during the overetch of the SiNx, especially at regions where the SiOx overlays steps formed by layers of underlying array components, such as edges of the source and drain metal or gate metal used in TFT formation. Accordingly, it is desirable to improve the sealing of the contact finger lines by the passivation of dielectric layers overlying the contact finger lines.
In addition, the contact vias in the contact pads in such devices do not inherently include a desired sloped profile or sides even though they are normally wet etched. At the relatively low temperature (about 250.degree. C.) used in the PECVD of SiOx, the resultant contact via profile often has a vertical sidewall profile or even an overhanging profile after wet etch in an HF-based solution, typically a 10% solution of 10% HF buffered with NH.sub.4 F. An overhang has been observed not only with the use of tank wet etch, but also with the use of spin and spray tools. The resultant overhang increases the difficulty of forming an electrically continuous layer of ITO across the edges of the via from the top surface of the contact pad to the underlying metals which connect to the photodiode array. Since flexible external connectors only contact the top surfaces of the contact pads, discontinuous ITO across the via edges results in uncontacted lines. Because an imager may have thousands of conducting lines, the ITO must make a very reliable connection across the via sidewall. Methods such as via bonding which would contact the bottom of the vias are not practical because of the thousands of bonds required.
It is thus desirable to provide an improved method of fabricating the electrical contact for a photosensitive element in which etching of the diode passivation layer does not adversely affect the underlying thin film transistor passivation dielectric and which provides improved yield and reliability of the common electrode contact to the underlying conductors, and in which the SiNx used as a diode passivation dielectric is removed from the contact finger region surrounding the array in order to improve the adherence of gasket materials used to seal the array.